1. Field of the Invention
This invention relates to an optical path folding apparatus for optical scanner and particularly to an optical path folding apparatus that has a double side reflecting mirror.
2. Description of the Prior Art
Optical scanner employs a principle of using an optical path folding apparatus to reflect light to a lens for forming a light image, then using a charged coupled device (CCD) to convert the light image to digital signals for computer process. Optically, it needs a certain length of optical path to form the light image required. If the optical path is stretched linearly, the optical path folding apparatus will become too bulky and is against the contemporary design concept of small size and light weight for electronic devices. Hence the optical path folding apparatus used in the contemporary scanners mostly uses a plurality of reflecting mirrors to squeeze the overall size while still get the optical length needed. The number, size and configuration of the reflecting mirrors directly affect the size and weight of the optical path folding apparatus and indirectly affect the size and weight of the scanner.
Most conventional scanners have four pieces of reflecting mirrors in the optical path folding apparatus. Each reflecting mirror does one light receiving and reflection. This four-mirror structure is bulky, heavy and costs higher. It thus does not suit well with the contemporary design trend set forth above.
Taiwan (R.O.C.) patent application number 79207828 (hereinafter will be referred to as "Prior Art 1") discloses an optical path folding apparatus for scanner that includes three pieces of reflecting mirrors. It has four light reflections with two light reflections taking place on the first reflection mirror. While it reduces somewhat the size and weight of the optical path folding apparatus, the two light reflections on the first reflecting mirror do not fall on the same spot. Therefore the size of the first reflecting mirror has to be increased which undercuts size and weight shrinking effect. Taiwan (R.O.C.) patent application number 85211358 (hereinafter will be referred to as "Prior Art 2") discloses another optical path folding apparatus which has three pieces of reflecting mirrors. Through arranging the position and angle of the reflecting mirrors, at least one mirror has two or more reflections taking place on same spot with different light incident angles. It has smaller size than that of the Prior Art 1. However the Prior Art 2 creates a new problem, i.e., producing diffracting light when incident light falling on the same spot of the reflecting mirror. It will downgrade scanning quality. Therefore there is still room for improvement regarding the optical, path folding apparatus.